Exit Pupil Expanders with Wide Field-of-View

ABSTRACT

The specification and drawings present a new apparatus and method for providing a wide field-of-view as well as illumination uniformity in exit pupil expanders (EPE) using stacked EPE substrates (or plates) with non-symmetric exit pupil expansion that use a plurality of diffractive elements for expanding the exit pupil of a display for viewing.

TECHNICAL FIELD

The present invention relates generally to display devices and, morespecifically, to providing a wide field-of-view as well as illuminationuniformity in exit pupil expanders that use a plurality of diffractiveelements for expanding the exit pupil of a display for viewing.

BACKGROUND ART

The Field-of-View (FOV) of diffractive Exit Pupil Expanders (EPEs) usedwith Near-to-Eye Displays (NEDs) is typically limited by the refractiveindex of the available EPE substrate materials. When used withpolychromatic light, and neglecting the effect of a display aspectratio, the horizontal FOV limit can be expressed as θ_(H) is a halfangle of the FOV):

$\begin{matrix}{{{\sin \; \theta_{H}} = \frac{{\lambda_{\min}n_{2}} - {\lambda_{\max}n_{1}}}{n_{1}\left( {\lambda_{\min} + \lambda_{\max}} \right)}},} & {(1),}\end{matrix}$

wherein n₂ is the refractive index of the EPE substrate (or plate), n₁is the refractive index of the surrounding material, and λ_(min) andλ_(max) are minimum and maximum wavelengths of the polychromatic light,respectively. Equation 1 is valid for symmetric exit pupil expansion,i.e., the grating period of the in-coupling diffraction grating isselected so that the horizontal acceptance angles for light guiding areequal for both +1 and −1 diffraction orders.

Equation 1 shows that the FOV of a planar EPE is limited by therefractive index of the materials and the wavelength band of theincident light. For example, using EPE substrate material MGC171(manufactured by MITSUBISHI GAS CHEMICALS) with the refractive index ofn₂=1.71 and n₁=1 (for air as the surrounding material) we get atheoretical FOV limit of about 40 degrees (full width equal to 2θ_(H))for blue light (λ=465 nm) having 10 nm wavelength bandwidth. When usingone EPE plate for blue (465 nm) and green (540 nm) light components anda substrate with n₂=1.71, the FOV is reduced to about 29 degrees. If ahigh index material of n₂=2 is used, the FOV limit for the 10 nmwavelength bandwidth and blue light (465 nm) is about 58 degrees, butfor the wavelength band covering the visible spectra (λ_(max)=450 nm andλ_(max)=650 nm) the FOV limit is only about 26 degrees.

Looking at the examples presented herein, it is clear that separate EPEplates with refractive index approaching n=2 are required for each RGB(red, green, blue) color to reach the viewing conditions of a typical PCdesktops monitor. In practice, such materials are not readily availableso other operating principles are needed.

DISCLOSURE OF THE INVENTION

According to a first aspect of the invention, an apparatus, comprises: afirst substrate of optical material having a first surface and a secondsurface which is opposite to the first surface; an in-couplingdiffractive element disposed on the first or the second surface of thefirst substrate; one or more out-coupling diffractive elements disposedon the first or the second surface, wherein the in-coupling diffractiveelement is configured to diffract optical rays of the input opticalbeam, which are incident from one half space relative to a planeperpendicular to the first substrate and parallel to periodic lines ofthe in-coupling diffractive element, so as to provide one diffractedoptical beam substantially within the first and second surfaces suchthat at least a portion of the one diffracted optical beam is coupledonly to one of the one or more out-coupling diffractive elements; asecond substrate of optical material having a further first surface anda further second surface and being positioned substantially parallel tothe first substrate and in juxtaposed spaced relationship hereto,wherein the further second surface is opposite to the further firstsurface; a further in-coupling diffractive element disposed on thefurther first or the further second surface substantially in alignmentwith the in-coupling diffractive element and having further periodiclines parallel to the periodic lines of the in-coupling diffractiveelement, and configured to receive a portion of the input optical beamwhich propagates through the first substrate into the further substrate;and one or more further out-coupling diffractive elements disposed onthe further first or the further second surface and each beingsubstantially opposite to a corresponding diffractive element of the oneor more out-coupling diffractive elements, wherein the furtherin-coupling diffractive element is configured to diffract optical raysof the portion of the input optical beam, which are incident fromanother half space relative to the plane, to provide a further onediffracted optical beam substantially within the further first andfurther second surfaces such that at least a portion of the furtherdiffracted optical beam is coupled only to one of the one or morefurther out-coupling diffractive elements which is opposite to the oneof the one or more out-coupling diffractive elements, and wherein theone of the one or more out-coupling diffractive elements is configuredto couple by diffraction parts of the one diffracted optical beam fromthe first substrate for providing an output optical beam out of thefirst substrate with an expanded exit pupil in one or two dimensions,the output optical beam being propagated through the second substrate,and the one of the one or more further out-coupling diffractive elementsis configured to couple by diffraction parts of the further onediffracted optical beams from the second substrate for providing afurther output optical beam out of the second substrate with theexpanded exit pupil in the one or two dimensions.

According further to the first aspect of the invention, the in-couplingdiffractive element may be further configured to diffract optical raysof the input optical beam, which are incident from another half spacerelative to the plane, to provide another diffracted optical beamsubstantially within the first and second surfaces such that at least aportion of the another diffracted optical beam is coupled only toanother of the one or two out-coupling diffractive elements, wherein thefurther in-coupling diffractive element may be configured to diffractoptical rays of the portion of the input optical beam, which areincident from the one half space relative to the plane, to provide afurther another diffracted optical beam substantially within the furtherfirst and further second surfaces such that at least a portion of thefurther another diffracted optical beam is coupled only to another ofthe one or two further out-coupling diffractive elements which isopposite to the another of the one or more out-coupling diffractiveelements, wherein the one and another of the one or more out-couplingdiffractive elements may be configured to couple by diffraction parts ofthe one and another diffracted optical beams from the first substratefor providing two output optical beams out of the first substrate withthe expanded exit pupil in one or two dimensions, the output opticalbeams being propagated through the second substrate, and the one andanother of the further one or more diffractive elements may beconfigured to couple by diffraction parts of the further one and furtheranother diffracted optical beams from the second substrate for providingtwo further output optical beams out of the second substrate with theexpanded exit pupil in the one or two dimensions. Still further, each ofthe two output optical beams and a corresponding each of the two furtheroutput optical beams may substantially coincide at a predetermineddistance range from the second substrate. Yet still further, the one andanother of the one or more out-coupling diffractive elements may besymmetrical relative to the in-coupling diffractive element and the oneand another of the one or more further out-coupling diffractive elementsmay be symmetrical relative to the further in-coupling diffractiveelement. Yet further still, distances from the one and another of theone or more out-coupling diffractive elements to the in-couplingdiffraction element may be different than corresponding distances fromthe one and another of the one or more further out-coupling diffractiveelements to the further in-coupling diffractive element.

Further according to the first aspect of the invention, optical rays ofthe output optical beam and of the further output optical beam may besubstantially parallel to corresponding optical rays of the inputoptical beam.

Still further according to the first aspect of the invention, when usingidentical materials and identical surrounding material for the first andsecond substrates, the in-coupling diffractive element may have a periodof the periodic lines equal to a maximum wavelength of the input opticalbeam divided by an index of refraction of the first and secondsubstrates, and the further in-coupling diffractive element may have afurther period of the further periodic lines equal to a minimumwavelength of the input optical beam divided by an index of refractionof a surrounding material of the first and second substrates.

According further to the first aspect of the invention, when usingidentical materials and identical surrounding material for the first andsecond substrates, the further in-coupling diffractive element may havea further period of the further periodic lines equal to a maximumwavelength of the input optical beam divided by an index of refractionof the first and second substrates, and the in-coupling diffractiveelement may have a period of the periodic lines equal to a minimumwavelength of the input optical beam divided by an index of refractionof a surrounding material of the first and second substrates.

According still further to the first aspect of the invention, a width ofthe in-coupling diffraction element in a direction perpendicular to theperiodic lines may be different from a corresponding width of thefurther in-coupling diffractive element in a direction perpendicular tothe further periodic lines.

According further still to the first aspect of the invention, the two ormore out-coupling diffractive elements, the in-coupling diffractiveelement, the one or more further out-coupling diffractive elements andthe further in-coupling diffractive element may have parallel periodiclines.

According yet further still to the first aspect of the invention, theapparatus may further comprise one or more further substrates,positioned substantially parallel to the first and second substrates andin juxtaposed spaced relationship thereto, with in-coupling andout-coupling diffractive elements disposed on respective surfaces of theone or more further substrates, wherein each of the one or more furthersubstrates with the disposed diffractive elements may be substantiallyidentical to the first substrate with the in-coupling diffractiveelement and the one or more out-coupling diffraction element or to thesecond substrate with the further in-coupling diffractive element andthe one or more further out-coupling diffraction elements.

Yet still further according to the first aspect of the invention, theapparatus may further comprise one or more further substrates,positioned substantially parallel to the first and second substrates andin juxtaposed spaced relationship thereto, with in-coupling andout-coupling diffractive elements disposed on respective surfaces of theone or more further substrates, wherein each of the one or more furthersubstrates with the disposed diffractive elements may be configured toperform a non-symmetric exit pupil extension for larger incidence anglesof the input optical beam than the first substrate with the in-couplingdiffractive element and the one or more out-coupling diffraction elementor the second substrate with the further in-coupling diffractive elementand the one or more further out-coupling diffraction elements.

Still yet further according to the first aspect of the invention, theapparatus may further comprise one or more further substrates,positioned substantially parallel to the first and second substrates andin juxtaposed spaced relationship thereto, with in-coupling andout-coupling diffractive elements disposed on respective surfaces of theone or more further substrates, wherein each of the one or more furthersubstrates with the disposed diffractive elements may be configured toperform a symmetric exit pupil extension only for smaller incidenceangles of the input optical beam than the first substrate with thein-coupling diffractive element and the one or more out-couplingdiffraction element or the second substrate with the further in-couplingdiffractive element and the one or more further out-coupling diffractionelements.

Still yet further still according to the first aspect of the invention,a material surrounding the first and second substrates may be air.

According still further to the first aspect of the invention, theapparatus may further comprise one or more intermediate diffractiveelements disposed on the first substrate and one or more furtherintermediate diffractive elements disposed on the second substrate suchthat at least parts of the input optical beam diffracted in thein-coupling diffractive element and in the further in-couplingdiffractive element may be first coupled to corresponding the one ormore intermediate diffractive elements and the one or more furtherintermediate diffractive elements, which are configured to furthercouple by diffraction corresponding optical beams to the one or moreout-coupling diffractive elements and to the one or more furtherout-coupling diffractive elements, for providing one or more outputoptical beams and one or more further output optical beams with theexpanded exit pupil in the two dimensions.

Further according to the first aspect of the invention, the inputoptical beam may be emanated from a virtual image of a display or amicrodisplay.

Still further according to the first aspect of the invention, grooves ofthe in-coupling diffractive element or the further in-couplingdiffractive element may have an asymmetric groove shape and may beslanted gratings.

According to a second aspect of the invention, a method, comprises:receiving an input optical beam by an in-coupling diffractive elementdisposed on a first or a second surface of a first substrate, whereinthe second surface is opposite to the first surface; diffracting opticalrays of the input optical beam, which are incident from one half spacerelative to a plane perpendicular to the first substrate and parallel toperiodic lines of the in-coupling diffractive element, using thein-coupling diffractive element disposed on the first or the secondsurface, so as to provide one diffracted optical beam substantiallywithin the first and second surfaces such that at least a portion of theone diffracted optical beam is coupled only to one of one or moreout-coupling diffractive elements disposed on the first or the furthersecond surface of the first substrate; coupling by diffraction parts ofthe one diffracted optical beams from the first substrate using the oneof the one or more out-coupling diffractive elements for providing anoutput optical beam out of the first substrate with an expanded exitpupil in one or two dimensions, and propagating the output optical beamthrough a second substrate, the second substrate being positionedsubstantially parallel to the first substrate and in juxtaposed spacedrelationship hereto; receiving a portion of the input optical beam whichpropagates through the first substrate into the second substrate, by afurther in-coupling diffractive element disposed on a further first or afurther second surface of the second substrate substantially inalignment with the in-coupling diffractive element and having furtherperiodic lines parallel to the periodic lines of the in-couplingdiffractive element, wherein the further second surface is opposite tothe further first surface; diffracting optical rays of the portion ofthe input optical beam, which are incident from the another half spacerelative to the plane, using the further in-coupling diffractive elementto provide a further one diffracted optical beam substantially withinthe further first and further second surfaces such that at least aportion of the further one diffracted optical beam is coupled only toone of the one or more further out-coupling diffractive elements whichis opposite to the one of the one or more out-coupling diffractiveelements; and coupling by diffraction parts of the further onediffracted optical beam from the second substrate using the one of theone or more further out-coupling diffractive elements for providing afurther output optical beam out of the second substrate with theexpanded exit pupil in the one or two dimensions.

According further to the second aspect of the invention, the method mayfurther comprise: diffracting optical rays of the input optical beam,which are incident from another half space relative to the plane, toprovide another diffracted optical beam substantially within the firstand second surfaces such that at least a portion of the anotherdiffracted optical beam is coupled only to another of the one or twoout-coupling diffractive elements, wherein the another of the one ormore out-coupling diffractive elements disposed on the first or thesecond surface of the first substrate; coupling by diffraction parts ofthe another diffracted optical beam from the first substrate usinganother of the one or more out-coupling diffractive elements forproviding another output optical beam out of the first substrate with anexpanded exit pupil in one or two dimensions, and propagating theanother output optical beam through the second substrate; diffractingoptical rays of the portion of the input optical beam, which areincident from the one half space relative to the plane, using thefurther in-coupling diffractive element to provide a further anotherdiffracted optical beam substantially within the further first andfurther second surfaces such that at least a portion of the furtheranother diffracted optical beam is coupled only to another of the one ormore further out-coupling diffractive elements which is opposite to theanother of the one or more out-coupling diffractive elements; andcoupling by diffraction parts of the further another diffracted opticalbeam from the second substrate using the another of the one or morefurther out-coupling diffractive elements for providing a furtheranother output optical beam out of the second substrate with theexpanded exit pupil in the one or two dimensions. Still further, the oneand another of the one or more out-coupling diffractive elements may besymmetrical relative to the in-coupling diffractive element and the oneand another of the one or more further out-coupling diffractive elementsmay be symmetrical relative to the further in-coupling diffractiveelement. Yet still further, distances from the one and another of theone or more out-coupling diffractive elements to the in-couplingdiffraction element may be different than corresponding distances fromthe one and another of the one or more further out-coupling diffractiveelements to the further in-coupling diffractive element.

Further according to the second aspect of the invention, when usingidentical materials and identical surrounding material for the first andsecond substrates, the in-coupling diffractive element may have a periodof the periodic lines equal to a maximum wavelength of the input opticalbeam divided by an index of refraction of the first and secondsubstrates, and the further in-coupling diffractive element may have afurther period of the further periodic lines equal to a minimumwavelength of the input optical beam divided by an index of refractionof a surrounding material of the first and second substrates or, whenusing identical materials and identical surrounding material for thefirst and second substrates, the further in-coupling diffractive elementmay have a further period of the further periodic lines equal to amaximum wavelength of the input optical beam divided by an index ofrefraction of the first and second substrates, and the in-couplingdiffractive element may have a period of the periodic lines equal to aminimum wavelength of the input optical beam divided by an index ofrefraction of a surrounding material of the first and second substrates.

Still further according to the second aspect of the invention, the twoor more out-coupling diffractive elements, the in-coupling diffractiveelement, the one or more further out-coupling diffractive elements andthe further in-coupling diffractive element may have parallel periodiclines.

According to a third aspect of the invention, an electronic device,comprises:

-   -   a data processing unit;    -   an optical engine operatively connected to the data processing        unit for receiving image data from the data processing unit;    -   a display device operatively connected to the optical engine for        forming an image based on the image data; and    -   an exit pupil expander comprising: a first substrate of optical        material having a first surface and a second surface which is        opposite to the first surface; an in-coupling diffractive        element disposed on the first or the second surface of the first        substrate; one or more out-coupling diffractive elements        disposed on the first or the second surface, wherein the        in-coupling diffractive element is configured to diffract        optical rays of the input optical beam, which are incident from        one half space relative to a plane perpendicular to the first        substrate and parallel to periodic lines of the in-coupling        diffractive element, so as to provide one diffracted optical        beam substantially within the first and second surfaces such        that at least a portion of the one diffracted optical beam is        coupled only to one of the one or more out-coupling diffractive        elements; a second substrate of optical material having a        further first surface and a further second surface and being        positioned substantially parallel to the first substrate and in        juxtaposed spaced relationship hereto, wherein the further        second surface is opposite to the further first surface; a        further in-coupling diffractive element disposed on the further        first or the further second surface substantially in alignment        with the in-coupling diffractive element and having further        periodic lines parallel to the periodic lines of the in-coupling        diffractive element, and configured to receive a portion of the        input optical beam which propagates through the first substrate        into the further substrate; and one or more further out-coupling        diffractive elements disposed on the further first or the        further second surface and each being substantially opposite to        a corresponding diffractive element of the one or more        out-coupling diffractive elements, wherein the further        in-coupling diffractive element is configured to diffract        optical rays of the portion of the input optical beam, which are        incident from another half space relative to the plane, to        provide a further one diffracted optical beam substantially        within the further first and further second surfaces such that        at least a portion of the further diffracted optical beam is        coupled only to one of the one or more further out-coupling        diffractive elements which is opposite to the one of the one or        more out-coupling diffractive elements, and wherein the one of        the one or more out-coupling diffractive elements is configured        to couple by diffraction parts of the one diffracted optical        beam from the first substrate for providing an output optical        beam out of the first substrate with an expanded exit pupil in        one or two dimensions, the output optical beam being propagated        through the second substrate, and the one of the one or more        further out-coupling diffractive elements is configured to        couple by diffraction parts of the further one diffracted        optical beams from the second substrate for providing a further        output optical beam out of the second substrate with the        expanded exit pupil in the one or two dimensions.

According further to the third aspect of the invention, the in-couplingdiffractive element may be further configured to diffract optical raysof the input optical beam, which are incident from another half spacerelative to the plane, to provide another diffracted optical beamsubstantially within the first and second surfaces such that at least aportion of the another diffracted optical beam is coupled only toanother of the one or two out-coupling diffractive elements, wherein thefurther in-coupling diffractive element may be configured to diffractoptical rays of the portion of the input optical beam, which areincident from the one half space relative to the plane, to provide afurther another diffracted optical beam substantially within the furtherfirst and further second surfaces such that at least a portion of thefurther another diffracted optical beam is coupled only to another ofthe one or two further out-coupling diffractive elements which isopposite to the another of the one or more out-coupling diffractiveelements, wherein the one and another of the one or more out-couplingdiffractive elements may be configured to couple by diffraction parts ofthe one and another diffracted optical beams from the first substratefor providing two output optical beams out of the first substrate withthe expanded exit pupil in one or two dimensions, the output opticalbeams being propagated through the second substrate, and the one andanother of the further one or more diffractive elements may beconfigured to couple by diffraction parts of the further one and furtheranother diffracted optical beams from the second substrate for providingtwo further output optical beams out of the second substrate with theexpanded exit pupil in the one or two dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the presentinvention, reference is made to the following detailed description takenin conjunction with the following drawings, in which:

FIGS. 1 a and 1 b are schematic representations of a symmetric exitpupil expander in a Near-to-Eye Display (NED): a general 3-dimensionalview is shown in FIG. 1 a, and a cross sectional view demonstratingpropagation of optical rays of the input optical beams from left andright half space is shown in FIG. 1 b;

FIG. 2 is a cross-sectional view of a non-symmetric exit pupil expanderdemonstrating propagation of optical rays of the input optical beamsfrom left and from right half space, according to an embodiment of thepresent invention;

FIG. 3 is a schematic representation (cross-sectional view)demonstrating a geometry of in-coupling diffractive elements(diffraction gratings) implemented as slanted asymmetric gratings,according to an embodiment of the present invention;

FIGS. 4 a-4 d are graphs of simulated efficiencies of in-couplingdiffraction gratings of FIG. 3 with 50 degrees slanted angle incorresponding left and right regions as a function of a incidence angle,according to an embodiment of the present invention;

FIGS. 5 a and 5 b are schematic representations (cross sectional views)demonstrating improving of out-coupling efficiency of a non-symmetricexit-pupil expander designed according to an embodiment of the presentinvention as shown in FIG. 5 b compared to out-coupling efficiency of asymmetric exit-pupil expander shown in FIG. 5 a,

FIG. 6 is a schematic representation (cross sectional views)demonstrating improving of in-coupling efficiency of a non-symmetricexit-pupil expander having multiple stacked substrates, according to anembodiment of the present invention;

FIGS. 7 a and 7 b are schematic representations (top views) of one area(left or right) of a two-dimensional diffractive exit pupil expander,wherein an intermediate diffractive element (grating) has an odd numberof first order diffractions (shown in FIG. 7 a) or an even number offurther first order reflections (shown in FIG. 7 b), according toembodiments of the present invention;

FIG. 8 is a flow chart demonstrating propagation of optical rays of theinput optical beams from left and right half spaces of a non-symmetricexit pupil expander, according to an embodiment of the presentinvention; and

FIG. 9 is a schematic representation of an electronic device, having anexit pupil expander system, according to embodiments of the presentinvention.

MODES FOR CARRYING OUT THE INVENTION

A new method and apparatus are presented for providing a widefield-of-view as well as for improving illumination uniformity in ExitPupil Expanders (EPE) using stacked EPE substrates (or plates) withnon-symmetric exit pupil expansion that use a plurality of diffractiveelements for expanding the exit pupil of a display for viewing. Theembodiments of the present invention can be applied to a broad opticalspectral range of optical beams but most importantly to a visible partof the optical spectrum where the optical beams are called light beams.It is further noted that for describing various embodiments of thepresent invention the term “substrate” can be interpreted as a thinplate with two flat or non-flat surfaces (e.g., first and secondsurfaces) parallel and opposite to each other. All examples fordifferent embodiments of the present invention provided herein are forflat substrates but in principle these embodiments can be applied tonon-flat stacked EPE substrates as well. Various embodiments of thepresent invention enable to increase field of view without the need forhigh refractive index materials. Alternatively, the embodiments of thepresent invention enable reduction of the refractive index requirementto reach a given FOV as compared with traditional symmetric EPE plate.This would allow utilization of readily available materials with lowerprice and improved optical/environmental characteristics as comparedwith more exotic substrate materials such as MGC171 (manufactured byMITSUBISHI GAS CHEMICALS) with the refractive index of 1.71.Furthermore, improvement of the illumination uniformity of the virtualdisplay can be also achieved using embodiments of the present invention.

According to an embodiment of the present invention, an optical device(e.g., the optical device can be a part of a virtual reality display)such as an exit pupil expander can comprise two (or more) substrates ofoptical material. We consider first a non-symmetric EPE comprising twosubstrates, wherein a first substrate has a first surface and a secondsurface (opposite to the first surface) and a second substrate has afurther first surface and a further second surface (opposite to thefurther first surface) and being positioned substantially parallel tothe first substrate and in juxtaposed spaced relationship hereto.

Moreover, the input optical beam emanated from an object or a virtualobject (e.g., a virtual image of a display or a microdisplay) can bereceived by an in-coupling diffractive element disposed on the first orthe second surface of the first substrate. Then optical rays of theinput optical beam, which are incident from one half space relative to aplane perpendicular to the first substrate and parallel to periodiclines of the in-coupling diffractive element (e.g., diffractiongrating), can be diffracted by the in-coupling diffractive element toprovide one diffracted optical beam substantially within the first andsecond surfaces of the first substrate such that at least a portion ofthe one diffracted optical beam is coupled (e.g., using total internalreflection in the first substrate and optionally using an intermediatediffraction grating for two-dimensional expansion) only to one of one ormore out-coupling diffractive elements (e.g., one out-couplingdiffractive element for a monocular viewing, two out-couplingdiffractive element for a binocular viewing, etc.) disposed on the firstor the second surface; and optical rays of the input optical beam, whichare incident from another half space relative to said plane, can befurther diffracted to provide another diffracted optical beamsubstantially within the first and second surfaces such that at least aportion of said another diffracted optical beam is coupled (e.g., usingtotal internal reflection in the first substrate and optionally usinganother intermediate diffraction grating) only to another (the binocularviewing is considered in this example) of the one or more out-couplingdiffractive elements. Then parts of said one and another diffractedoptical beams can be coupled by diffraction from the first substrateusing the one or more out-coupling diffractive elements for providingtwo output optical beams out of the first substrate with an expandedexit pupil in one or two dimensions (two-dimension expansion is providedby using intermediate diffraction gratings), and propagating said outputoptical beams through the second substrate to viewer eyes.

Furthermore, a portion of the original input optical beam from theobject or the virtual object (e.g., the virtual image of the display orthe microdisplay), which propagates through said first substrate (e.g.,without changing a direction) into the second substrate, can be receivedby a further in-coupling diffractive element disposed on the furtherfirst or further second surface of the second substrate andsubstantially in alignment with the in-coupling diffractive element ofthe first substrate and having further periodic lines parallel to theperiodic lines of the in-coupling diffractive element. Then optical raysof the portion of the input optical beam, which are incident from theone half space relative to said plane, can be diffracted by the furtherin-coupling diffractive element to provide a further one diffractedoptical beam substantially within said further first and further secondsurfaces such that at least a portion of said further one diffractedoptical beam is coupled (e.g., using the total internal reflection inthe second substrate and optionally a further intermediate diffractiongrating) only to one of one or more further out-coupling diffractiveelements (e.g., one further out-coupling diffractive element for amonocular viewing, two further out-coupling diffractive element for abinocular viewing, etc.) which is opposite to said another of the one ormore out-coupling diffractive elements of the first substrate, andoptical rays of the portion of the input optical beam, which areincident from the another half space relative to said plane, can bediffracted using the further in-coupling diffractive element to providea further another diffracted optical beam substantially within thefurther first and further second surfaces of the second substrate suchthat at least a portion of the further another diffracted optical beamis coupled (e.g., using total internal reflection in the secondsubstrate and optionally another further intermediate diffractiongrating) only to another (the binocular viewing is considered in thisexample, as stated herein) of the one or more further out-couplingdiffractive elements which is opposite to said one of the one or moreout-coupling diffractive elements of the first substrate. Then parts ofsaid further one and further another diffracted optical beams from thesecond substrate can be coupled using the one or more furtherout-coupling diffractive elements for providing two further outputoptical beams out of the second substrate with the expanded exit pupilin the one or two dimensions (two-dimension expansion is provided byusing intermediate diffraction gratings) directly to the viewer eyes.

It is noticed that in the scenario described above, each of the twooutput optical beams and a corresponding each of the two further outputoptical beams can substantially coincide at a predetermined distancerange from the second substrate, thus providing an image to both viewereyes with a wide field-of-view compared to a one-substrate case (seediscussion in regard to FIGS. 1 and 2).

Also as known in the art for EPE for the flat substrates, typicallyoptical rays of the output optical beams and of the further outputoptical beams are substantially parallel to corresponding optical raysof the input optical beam.

It is further noticed that the virtual display can be monocular orbinocular, therefore the scenario described herein can be used for bothmonocular and binocular viewing. Also, according to a further embodimentof the present invention, the EPE with stacked substrates can bedesigned only for monocular viewing with one out-coupling gratingelement in each substrate, wherein these out-coupling grating elementscan be substantially opposite to each other and located in the same halfsphere relative to a plane perpendicular to the substrates and parallelto periodic lines of the in-coupling diffractive elements.

According to a further embodiment, wherein, when using identicalmaterials and identical surrounding material for the first and secondsubstrates, the in-coupling diffractive element of the first substratecan have a period of the periodic lines equal to a maximum wavelength(or alternatively equal to a minimum wavelength) of the input opticalbeam divided by an index of refraction of the first and secondsubstrates (alternatively by an index of refraction of a surroundingmaterial of the first and second substrates), and the furtherin-coupling diffractive element of the second substrate can have afurther period of the further periodic lines equal to a minimumwavelength (alternatively equal to a minimum wavelength) of the inputoptical beam divided by an index of refraction of a surrounding materialof the first and second substrates (or alternatively by an index ofrefraction of the first and second substrates). The surrounding materialof the first or the second substrate can be air with index of refractionof 1.

Moreover, if two out-coupling diffractive elements are used in eachsubstrate, typically the two out-coupling diffractive elements can besymmetrical relative to the in-coupling diffractive element in eachsubstrate. Also, the out-coupling diffractive elements and thein-coupling diffractive elements in each substrate can have parallelperiodic lines.

There are many further possible variations. For example, distances fromthe one or more out-coupling diffractive elements to the in-couplingdiffraction element in the first substrate may be the same as ordifferent from (see discussions in reference to FIGS. 5 a and 5 b) thedistances from the corresponding one or more further out-couplingdiffractive elements to the further in-coupling diffractive element inthe second substrate in order to provide better efficiency anduniformity.

Also, a width of the in-coupling diffraction element of the firstsubstrate in a direction perpendicular to the periodic lines may bedifferent from a corresponding width of the further in-couplingdiffractive element of the second substrate in a direction perpendicularto the further periodic lines to provide better coupling efficiency (seediscussions in reference to FIG. 5 c).

According to further embodiments, the two-dimensional expansion can beprovided by using one or more intermediate diffractive elements disposedon the first substrate and one or more further intermediate diffractiveelements disposed on the second substrate such that at least parts ofthe input optical beam diffracted in the in-coupling diffractive elementand in the further in-coupling diffractive element are first coupled tocorresponding the one or more intermediate diffractive elements and theone or more further intermediate diffractive elements, which can beconfigured to further couple by diffraction corresponding optical beamsto the one or more out-coupling diffractive elements and to the one ormore further out-coupling diffractive elements, respectively, forproviding the one or more output optical beams and one or more furtheroutput optical beams with the expanded exit pupil in two dimensions (seefurther discussions in reference to FIGS. 7 a and 7 b). The intermediatediffractive element can have an odd number of first order diffractionsor an even number of further first order reflections as known in the artand, e.g., described by T. Levola in “Diffractive Optics for VirtualReality Displays”, SID Eurodisplay 05, Edinburg (2005), SID 02 Digest,Paper 22.1.

According to embodiments of the present invention, the in-couplingdiffractive elements (or the in-coupling diffraction gratings) can beimplemented using a variety of different types of diffraction gratings,e.g., planar diffraction gratings manufactured using lithographicmethods or classically ruled (having different groove angles andprofiles, such as binary, triangular, sinusoidal, etc.). The diffractiveelements (i.e., their grooves) can be symmetric or asymmetric, e.g.,using slanted gratings for increasing the coupling efficiency andreducing an “optical crosstalk” between left and right half spaces. Forexample, the slanted gratings can be asymmetric such that their slantingangles are equal but have opposite signs relative to the optical axis ofthe input optical beam, i.e., the groove shapes are mirror images ofeach other. Thus, grooves of the in-coupling diffractive element or thefurther in-coupling diffractive element can have an asymmetric grooveshape and implemented, e.g., as slanted diffraction gratings (seefurther discussions in reference to FIGS. 3 and 4).

Furthermore, the EPE device can further comprise one or more furthersubstrates, positioned substantially parallel to the first and secondsubstrates, described herein, and in juxtaposed spaced relationshipthereto, with in-coupling and out-coupling diffraction elements disposedon respective surfaces of the one or more further substrates, whereineach of the one or more further substrates with the disposed diffractiveelements is substantially identical to the first substrate with thein-coupling diffractive element and the one or more out-couplingdiffraction elements or to the second substrate with the furtherin-coupling diffractive element and the one or more further out-couplingdiffraction elements.

As previously stated, the examples for different embodiments of thepresent invention provided herein are primarily for the flat substratesbut in principle these embodiments can be applied to non-flat stackedEPE substrates as well. The non-flat substrates can be cylindrical(e.g., see PCT patent application, International Publication NumberWO2006064301), spherical or aspheric as described in co-pending PCTapplication filed on the same date, docket number 944-16.21. Also theterm “aspheric” can be broadly defined as a surface with a profile thatis neither a portion of a sphere nor of a circular cylinder and it isnot flat and can be described by complex equations, wherein simpleexamples can include but are not limited to parabola, hyperbola,ellipse, etc.

FIGS. 1 a and 1 b shows an example of a schematic representations of asymmetric exit pupil expander (EPE) 1 in a Near-to-Eye Display (NED)application: a general 3-dimentional view is shown in FIG. 1 a, and across sectional view demonstrating propagation of optical rays of theinput optical beams from left and right half space is shown in FIG. 1 b.Input optical rays 5 and 6 emanated from a virtual image of a display(e.g., a microdisplay) 7 are coupled into a planar light guide plate(substrate) 4 using a diffraction grating 2. The in-coupled rayspropagate inside the light guide plate by a way of total internalreflection (TIR). Pupil expansion for the input image is achieved usinga second diffraction gratings 3-1 and 3-2 that couple out parts of thecoupled (trapped) light as output beams 5-6 and 6-5 to provide an imageof the microdisplay 7. For proper viewing conditions the wholeField-of-View (FOV) spanned by the input optical rays (φ_(h) is anincidence angle) must be coupled into the light guide plate by thein-coupling grating. Moreover, the whole FOV must be provided for botheyes, i.e., to left and right branches of the EPE 1. This means that theinput optical rays 6 incident from the right half space must be coupledto both the left and right eyes of the viewer, and the same must holdfor the input optical rays 5 incident from the left half space. Forimaging purposes it is advantageous to limit the in-coupling grating tosupport only first diffraction orders (m=±1). For the coordinate systemgiven in FIGS. 1 a and 1 b, the light diffracted to the m=+1 diffractionorder is coupled to the left branch of the EPE plate and the lightdiffracted to the m=−1 diffraction order is coupled to the right branch,respectively. Proper light in-coupling is achieved when the followingtwo conditions are met. First, the incidence angle of the in-coupledlight with respect to a plane normal of the light guide plate must belarge enough to support TIR. The incidence angle can be increased byreducing the grating period of the in-coupling grating. Second, thegrating period of the in-coupling grating must support first orderdiffraction, i.e., m=±1 diffraction orders (but not higher diffractionorders) must exist for incidence angles within the whole FOV. This willlimit the minimum grating period that can be used for given a material.By combining the two conditions it can be shown that the horizontal FOVlimit for given materials and polychromatic light is given by Equation1.

The approach discussed in reference to FIGS. 1 a and 1 b for symmetricexit pupil expander can be used for comparison with the case ofnon-symmetric exit pupil expander with stacked EPE substrate designedaccording to various embodiments of the present invention.

FIG. 2 shows one example among others of a cross-sectional view of anon-symmetric exit pupil expander 10 for demonstrating propagation ofoptical rays 16 and 18 of the input optical beams from the left andright half spaces, according to an embodiment of the present invention.

The operation of the non-symmetric EPE can be understood by consideringthe light in-coupling for rays that propagate along xz-plane, i.e.,horizontal rays. The grating period for in-coupling and out-couplingareas of the first EPE plate can be selected so that light from theright half space (θ_(i) is an incidence angle) is coupled to the lefteye of the viewer and the light from the left half space is coupled tothe right eye. The grating periods of the second EPE plate can beselected so that couplings are reversed.

For example, the grating period of the in-coupling grating 12-1 on thefirst EPE substrate (plate) 12 can be selected so that m=−1 diffractionorder can exist only for optical rays 18 incident from the right halfspace and m=+1 diffraction order can exist only for optical rays 16incident from the left half space. This means that optical rays 18incident from the right half space can be coupled only to the rightbranch of the first EPE substrate 12. Similarly, optical rays 18incident from the left half space can be coupled only to the left branchof the first EPE plate 12. For polychromatic illumination this can beachieved if the grating period of the in-coupling grating 12-1 of thefirst EPE substrate 12 is selected as

$\begin{matrix}{{d_{1} = \frac{\lambda_{\max}}{n_{2}}},} & (2)\end{matrix}$

wherein λ_(max) is a maximum wavelength of the polychromatic incidentlight and n₂ is the refractive index of the EPE substrates 12 and 14(assuming that these substrates are made from the same material).

Moreover, the grating period of the in-coupling grating on the secondEPE substrate 14 can be selected such that m=+1 diffraction order canexist only for optical rays 18 incident from the right half space andm=−1 diffraction order exist only for optical rays 16 incident from theleft half space. This means that optical rays 18 incident from the righthalf space can be coupled only to the left branch of the second EPEsubstrate 14. Similarly, optical rays 16 incident from the left halfspace can be coupled only to the right branch of the EPE substrate 14.For polychromatic illumination this is achieved if the grating period ofthe in-coupling diffraction grating 14-1 of the second EPE substrate 14is selected as

$\begin{matrix}{{d_{2} = \frac{\lambda_{\min}}{n_{1}}},} & (3)\end{matrix}$

wherein λ_(min) is a minimum wavelength of the polychromatic incidentlight and n₁ is the refractive index of surrounding material (e.g.,equal to 1 for air) of the EPE substrates 12 and 14 (assuming that thesesubstrates have the same surrounding material).

Thus, the viewer can see an output optical beams 20 and 22 coupled bycorresponding out-coupling diffraction gratings 12-2 a and 12-2 b fromthe first EPE substrate 12 and coupled by corresponding out-couplingdiffraction gratings 14-2 a and 14-2 b from the second EPE substrate 14.The FOV of the input optical beam that can be observed through the EPE10 can be determined by considering the combined criteria for theexistence of the diffraction orders as explained above and existence ofthe TIR inside the plates. This results for the horizontal FOV of thestacked non-symmetric EPE discussed herein can be expresses as follows:

$\begin{matrix}\begin{matrix}{w_{\lambda} = {\min \left( {\frac{{n_{2}\lambda_{\min}} - {n_{1}\lambda_{\max}}}{n_{1}\lambda_{\max}},\frac{{n_{2}\lambda_{\min}} - {n_{1}\lambda_{\max}}}{n_{1}\lambda_{\min}}} \right)}} \\{= {\frac{{n_{2}\lambda_{\min}} - {n_{1}\lambda_{\max}}}{n_{1}\lambda_{\max}}.}}\end{matrix} & (4)\end{matrix}$

Equation 4 shows that the non-symmetric EPE enables FOV improvement withrespect to the symmetric case (see Equation 1 and FIGS. 1 a and 1 b) bya factor of

$\begin{matrix}{M_{FOV} = \frac{\lambda_{\max} + \lambda_{\min}}{\lambda_{\max}}} & (5)\end{matrix}$

This results is an improvement by a factor of 1.69 for the visiblespectra (λ_(min)=450 nm and λ_(max)=650 nm) for the stackednon-symmetric EPE. For a narrow wavelength band, e.g. λ_(max)−λ_(min)=10nm, FOV improvement by factor approaching 2 can be obtained.

It is further noted that the order of EPE substrates 12 and 14 can bereversed according to another embodiment of the present invention, asstated herein.

Moreover, a further improvement of the FOV may be obtained by usingadditional one or more EPE plates or one or more EPE plate pairsdesigned to operate at larger complementary incidence angles outside ofthe angular acceptance range of the original non-symmetric EPE plates orEPE plate pairs.

It is further noted that in general the EPE stack need not be an evencombination of the EPE plates: for example it could an even or oddnumber of plates including, e.g., one or more symmetric EPE plates andtwo or more non-symmetric EPE plates (e.g., on or more EPE pairs)operating at incidence angles outside (i.e., larger than) the angularacceptance range of the one or more symmetric EPE plates.

FIG. 3 shows one example among others of a schematic representation(cross-sectional view) demonstrating a geometry of in-couplingdiffractive elements (diffraction gratings) 12-1 and 14-1 implemented asasymmetric slanted asymmetric gratings with 50 degrees slanting angle,according to an embodiment of the present invention. The grating,optical rays and substrate numbers correspond to the reference numbersof corresponding components shown in FIG. 2. Grooves of the in-couplingdiffractive elements 12-1 and 14-1 have grooves with an asymmetricgroove shape: 12-1 a and 14-1 a on the left in region 1 and 12-1 b and14-1 b on the right in region II, respectively.

FIGS. 4 a-4 d shows an example among others of graphs of simulated(using Rigorous Fourier Modal Method for multilayer surface reliefgratings) efficiencies ρ of the in-coupling slanted diffraction gratings12-1 and 14-1 shown in FIG. 3 with 50 degrees slanted angle incorresponding left and right regions (indicated in FIG. 3) as a functionof the incidence angle θ as defined herein, according to embodiments ofthe present invention. In FIGS. 4 a-4 d: φ is the angle between the xaxis (shown in FIGS. 1-3) and the projection of the incident directionvector, {circumflex over (k)}, of the input optical beam on the gratingsurface. Parameters of gratings 12-1 and 14-1 are the following: EPErefractive index n=1.71, surrounding material n=1, wavelength λ=525 nm,angle φ=0 degrees, polarization of light TM, grating period d=312 nm(for the grating 12-1), d=525 nm (for the grating 14-1), grating height300 nm, fill factor f=0.5 and slanting angle±50 degrees (region II/I).

FIGS. 5 a and 5 b show examples among others of schematicrepresentations (cross sectional views) demonstrating improving ofout-coupling efficiency of a non-symmetric exit-pupil expander designedaccording to an embodiment of the present invention as shown in FIG. 5 bcompared to out-coupling efficiency of a symmetric exit-pupil expandershown in FIG. 5 a. The grating, optical rays and substrate numberscorrespond to the reference numbers of corresponding components shown inFIGS. 1 a, 1 b and 2, respectively.

In a symmetric case shown in FIG. 5 a, out-coupling gratings (exit pupilsize) must span the whole FOV and thus cover the whole eye movementrange, wherein areas 8 represent “wasted” light. In non-symmetric caseshown in FIG. 5 b, according to an embodiment of the present invention,size of out-coupling gratings for each plate needs to span only half FOVand, thus, only half of the eye movement range. This allows for optimumpositioning of the out-coupling areas to reduce the amount of “wasted”light. Also in addition to the FOV increase, the non-symmetric expandercan be utilized to improve the illumination uniformity of the EPE. Asshown in FIG. 5 b, the positioning of the out-coupling gratings 12-2 a,12-2 b, 14-2 a and 14-2 b can be optimized separately for each halfspace with respect to left/right eye position. This enables improvementin the left/right eye illumination symmetry, illumination uniformity andreduction of unwanted light losses as described herein.

FIG. 6 shows an example among others of a schematic representation(cross sectional views) demonstrating improving of in-couplingefficiency of a non-symmetric exit-pupil expander having multiplestacked substrates (only in-coupling region is shown), according to anembodiment of the present invention. The grating, optical rays andsubstrate numbers correspond to the reference numbers of correspondingcomponents shown in FIG. 2. FIG. 6 shows the case when one or moreadditional EPE substrates are added compared to the 2-substratearrangement shown in FIGS. 2 and 3 (only substrate 30 with anin-coupling grating 30-1 is shown) to accommodate a broad wavelengthband. As a practical example, if a pair of non-symmetric EPE plates withn₂=1.71 is used for blue (465 nm) and green (540 nm) wavelengths and aseparate pair of non-symmetric EPE plates with n₂=1.71 is used for red(650 nm) wavelength, FOV value of about 60 degrees is reachable.Moreover, if a pair of non-symmetric EPE plates with n₂=1.71 is used foreach of the RGB components using a stack of six plates, FOV value ofabout 74 degrees should be possible. In addition, FIG. 6 demonstratethat an improvement in coupling efficiency can be achieved by increasingthe width of the in-coupling gratings 14-1 and 30-1 such that more lightcan be collected at the ends of the gratings 14-1 and 30-1.

FIGS. 7 a and 7 b show examples among others of schematicrepresentations (top views) of one area (left or right) of atwo-dimensional diffractive exit pupil expander according to embodimentsof the present invention, wherein an intermediate diffractive element(grating) 24 or 26 has an odd number of first order diffractions (shownin FIG. 7 a) or an even number of further first order reflections (shownin FIG. 7 b), as described by T. Levola in “Diffractive Optics forVirtual Reality Displays”, SID Eurodisplay 05, Edinburg (2005), SID 02Digest, Paper 22.1. The angle ρ is a rotation angle between the periodiclines of the intermediate diffraction grating 26 and the in-couplinggrating 12-1 or 14-1. respectively. The grating and substrate numbers inFIGS. 7 a and 7 b correspond to the reference numbers of correspondingcomponents shown in FIG. 2.

FIG. 8 shows a flow chart demonstrating propagation of optical rays ofthe input optical beams from left and right half space of anon-symmetric exit pupil expander, according to an embodiment of thepresent invention.

The flow chart of FIG. 8 only represents one possible scenario amongothers. It is noted that the order of steps shown in FIG. 8 is notabsolutely required, so in principle, the various steps can be performedout of order. In a method according to the embodiment of the presentinvention, in a first step 40, an input optical beam is received byin-coupling diffraction gratings of the first and second substrates.

In a next step 42, optical rays coming from different half spaces arediffracted by the in-coupling grating of the first substrate to only onecorresponding out-coupling grating (by TIR in the first substrate andoptionally using an intermediate grating in the first substrate fortwo-dimensional expansion). In a next step 44, the provided opticalcomponents are coupled out by the out-coupling diffraction gratings fromthe first substrate (the out-coupled optical components are propagatedthrough the second substrate to the viewer eyes) thus providing twooutput optical beams out of the second substrate with the expanded exitpupil in the one or two dimensions.

In a next step 46, optical rays coming from different half spaces arediffracted by the in-coupling grating of the second substrate to onlyone corresponding out-coupling grating (by TIR in the second substrateand optionally using an intermediate grating in the second substrate fortwo-dimensional expansion) which is opposite to correspondingout-coupling gratings of the first substrate. In a next step 48, theprovided optical components are coupled out by the out-coupling gratingsfrom the second substrate to the viewer eyes, providing two furtheroutput optical beams out of the second substrate with the expanded exitpupil in the one or two dimensions.

FIG. 9 is a schematic representation of an electronic device, having anexit pupil expander system, according to embodiments of the presentinvention.

FIG. 9 shows an example of a schematic representation of an electronicdevice 100, having the exit pupil expander (EPE) system 10 (10 a, or 10b) according to an embodiment of the present invention.

The exit pupil expander (EPE) 10, 10 a or 10 b can be used in anelectronic (portable) device 100, such as a mobile phone, personaldigital assistant (PDA), communicator, portable Internet appliance,hand-hand computer, digital video and still camera, wearable computer,computer game device, specialized bring-to-the-eye product for viewingand other portable electronic devices. As shown in FIG. 9, the portabledevice 100 has a housing 210 to house a communication unit 212 forreceiving and transmitting information from and to an external device(not shown). The portable device 100 also has a controlling andprocessing unit 214 for handling the received and transmittedinformation, and a virtual display system 230 for viewing. The virtualdisplay system 230 includes a micro-display or an image source 192 andan optical engine 190. The controlling and processing unit 214 isoperatively connected to the optical engine 190 to provide image data tothe image source 192 to display an image thereon. The EPE 10, accordingto the present invention, can be optically linked to an optical engine190.

Furthermore, the image source 192, as depicted in FIG. 9, can be asequential color LCOS (Liquid Crystal On Silicon) device, an OLED(Organic Light Emitting Diode) array, an MEMS (MicroElectro MechanicalSystem) device or any other suitable micro-display device operating intransmission, reflection or emission.

Moreover, the electronic device 100 can be a portable device, such as amobile phone, personal digital assistant (PDA), communicator, portableInternet appliance, hand-held computer, digital video and still camera,wearable computer, computer game device, specialized bring-to-the-eyeproduct for viewing and other portable electronic devices. However, theexit pupil expander, according to an embodiment of the presentinvention, can also be used in a non-portable device, such as a gamingdevice, vending machine, band-o-matic, and home appliances, such as anoven, microwave oven and other appliances and other non-portabledevices.

It is noted that various embodiments of the present invention recitedherein can be used separately, combined or selectively combined forspecific applications.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the scope ofthe present invention, and the appended claims are intended to coversuch modifications and arrangements.

1. An apparatus, comprising: a first substrate of optical materialhaving a first surface and a second surface which is opposite to thefirst surface; an in-coupling diffractive element disposed on the firstor the second surface of said first substrate; one or more out-couplingdiffractive elements disposed on the first or the second surface,wherein said in-coupling diffractive element is configured to diffractoptical rays of the input optical beam, which are incident from one halfspace relative to a plane perpendicular to said first substrate andparallel to periodic lines of the in-coupling diffractive element, so asto provide one diffracted optical beam substantially within said firstand second surfaces such that at least a portion of said one diffractedoptical beam is coupled only to one of said one or more out-couplingdiffractive elements; a second substrate of optical material having afurther first surface and a further second surface and being positionedsubstantially parallel to the first substrate and in juxtaposed spacedrelationship hereto, wherein the further second surface is opposite tothe further first surface; a further in-coupling diffractive elementdisposed on the further first or the further second surfacesubstantially in alignment with said in-coupling diffractive element andhaving further periodic lines parallel to said periodic lines of thein-coupling diffractive element, and configured to receive a portion ofsaid input optical beam which propagates through said first substrateinto the further substrate; and one or more further out-couplingdiffractive elements disposed on the further first or the further secondsurface and each being substantially opposite to a correspondingdiffractive element of said one or more out-coupling diffractiveelements, wherein said further in-coupling diffractive element isconfigured to diffract optical rays of the portion of the input opticalbeam, which are incident from another half space relative to said plane,to provide a further one diffracted optical beam substantially withinsaid further first and further second surfaces such that at least aportion of said further diffracted optical beam is coupled only to oneof said one or more further out-coupling diffractive elements which isopposite to said one of the one or more out-coupling diffractiveelements, and wherein said one of the one or more out-couplingdiffractive elements is configured to couple by diffraction parts ofsaid one diffracted optical beam from the first substrate for providingan output optical beam out of said first substrate with an expanded exitpupil in one or two dimensions, said output optical beam beingpropagated through said second substrate, and said one of the one ormore further out-coupling diffractive elements is configured to coupleby diffraction parts of said further one diffracted optical beams fromthe second substrate for providing a further output optical beam out ofsaid second substrate with said expanded exit pupil in said one or twodimensions.
 2. The apparatus of claim 1, wherein said in-couplingdiffractive element is further configured to diffract optical rays ofthe input optical beam, which are incident from another half spacerelative to said plane, to provide another diffracted optical beamsubstantially within said first and second surfaces such that at least aportion of said another diffracted optical beam is coupled only toanother of said one or two out-coupling diffractive elements, whereinsaid further in-coupling diffractive element is configured to diffractoptical rays of said portion of the input optical beam, which areincident from said one half space relative to said plane, to provide afurther another diffracted optical beam substantially within saidfurther first and further second surfaces such that at least a portionof said further another diffracted optical beam is coupled only toanother of said one or two further out-coupling diffractive elementswhich is opposite to said another of the one or more out-couplingdiffractive elements, wherein said one and another of the one or moreout-coupling diffractive elements are configured to couple bydiffraction parts of said one and another diffracted optical beams fromthe first substrate for providing two output optical beams out of saidfirst substrate with the expanded exit pupil in one or two dimensions,said output optical beams being propagated through said secondsubstrate, and said one and another of the further one or morediffractive elements are configured to couple by diffraction parts ofsaid further one and further another diffracted optical beams from thesecond substrate for providing two further output optical beams out ofsaid second substrate with the expanded exit pupil in said one or twodimensions.
 3. The apparatus of claim 2, wherein each of said two outputoptical beams and a corresponding each of said two further outputoptical beams substantially coincide at a predetermined distance rangefrom the second substrate.
 4. The apparatus of claim 2, wherein the oneand another of said one or more out-coupling diffractive elements aresymmetrical relative to the in-coupling diffractive element and the oneand another of the one or more further out-coupling diffractive elementsare symmetrical relative to the further in-coupling diffractive element.5. The apparatus of claim 2, wherein distances from the one and anotherof said one or more out-coupling diffractive elements to the in-couplingdiffraction element are different than corresponding distances from theone and another of the one or more further out-coupling diffractiveelements to the further in-coupling diffractive element.
 6. Theapparatus of claim 1, wherein optical rays of the output optical beamand of the further output optical beam are substantially parallel tocorresponding optical rays of the input optical beam.
 7. The apparatusof claim 1, wherein, when using identical materials and identicalsurrounding material for the first and second substrates, thein-coupling diffractive element has a period of the periodic lines equalto a maximum wavelength of the input optical beam divided by an index ofrefraction of the first and second substrates, and the furtherin-coupling diffractive element has a further period of the furtherperiodic lines equal to a minimum wavelength of the input optical beamdivided by an index of refraction of a surrounding material of the firstand second substrates.
 8. The apparatus of claim 1, wherein, when usingidentical materials and identical surrounding material for the first andsecond substrates, the further in-coupling diffractive element has afurther period of the further periodic lines equal to a maximumwavelength of the input optical beam divided by an index of refractionof the first and second substrates, and the in-coupling diffractiveelement has a period of the periodic lines equal to a minimum wavelengthof the input optical beam divided by an index of refraction of asurrounding material of the first and second substrates.
 9. Theapparatus of claim 1, wherein a width of the in-coupling diffractionelement in a direction perpendicular to the periodic lines is differentfrom a corresponding width of the further in-coupling diffractiveelement in a direction perpendicular to the further periodic lines. 10.The apparatus of claim 1, wherein the two or more out-couplingdiffractive elements, the in-coupling diffractive element, the one ormore further out-coupling diffractive elements and the furtherin-coupling diffractive element have parallel periodic lines.
 11. Theapparatus of claim 1, further comprising one or more further substrates,positioned substantially parallel to the first and second substrates andin juxtaposed spaced relationship thereto, with in-coupling andout-coupling diffractive elements disposed on respective surfaces ofsaid one or more further substrates, wherein each of said one or morefurther substrates with the disposed diffractive elements issubstantially identical to said first substrate with the in-couplingdiffractive element and the one or more out-coupling diffraction elementor to said second substrate with the further in-coupling diffractiveelement and the one or more further out-coupling diffraction elements.12. The apparatus of claim 1, further comprising one or more furthersubstrates, positioned substantially parallel to the first and secondsubstrates and in juxtaposed spaced relationship thereto, within-coupling and out-coupling diffractive elements disposed on respectivesurfaces of said one or more further substrates, wherein each of saidone or more further substrates with the disposed diffractive elements isconfigured to perform a non-symmetric exit pupil extension for largerincidence angles of said input optical beam than said first substratewith the in-coupling diffractive element and the one or moreout-coupling diffraction element or said second substrate with thefurther in-coupling diffractive element and the one or more furtherout-coupling diffraction elements.
 13. The apparatus of claim 1, furthercomprising one or more further substrates, positioned substantiallyparallel to the first and second substrates and in juxtaposed spacedrelationship thereto, with in-coupling and out-coupling diffractiveelements disposed on respective surfaces of said one or more furthersubstrates, wherein each of said one or more further substrates with thedisposed diffractive elements is configured to perform a symmetric exitpupil extension only for smaller incidence angles of said input opticalbeam than said first substrate with the in-coupling diffractive elementand the one or more out-coupling diffraction element or said secondsubstrate with the further in-coupling diffractive element and the oneor more further out-coupling diffraction elements.
 14. The apparatus ofclaim 1, wherein a material surrounding the first and second substratesis air.
 15. The apparatus of claim 1, further comprises one or moreintermediate diffractive elements disposed on the first substrate andone or more further intermediate diffractive elements disposed on thesecond substrate such that at least parts of the input optical beamdiffracted in the in-coupling diffractive element and in the furtherin-coupling diffractive element are first coupled to corresponding saidone or more intermediate diffractive elements and said one or morefurther intermediate diffractive elements, which are configured tofurther couple by diffraction corresponding optical beams to the one ormore out-coupling diffractive elements and to the one or more furtherout-coupling diffractive elements, for providing one or more outputoptical beams and one or more further output optical beams with saidexpanded exit pupil in said two dimensions.
 16. The apparatus of claim1, wherein said input optical beam is emanated from a virtual image of adisplay or a microdisplay.
 17. The apparatus of claim 1, wherein groovesof said in-coupling diffractive element or said further in-couplingdiffractive element have an asymmetric groove shape and are slantedgratings.
 18. A method, comprising: receiving an input optical beam byan in-coupling diffractive element disposed on a first or a secondsurface of a first substrate, wherein the second surface is opposite tothe first surface; diffracting optical rays of the input optical beam,which are incident from one half space relative to a plane perpendicularto said first substrate and parallel to periodic lines of thein-coupling diffractive element, using said in-coupling diffractiveelement disposed on the first or the second surface, so as to provideone diffracted optical beam substantially within said first and secondsurfaces such that at least a portion of said one diffracted opticalbeam is coupled only to one of one or more out-coupling diffractiveelements disposed on the first or the further second surface of thefirst substrate; coupling by diffraction parts of said one diffractedoptical beams from the first substrate using said one of said one ormore out-coupling diffractive elements for providing an output opticalbeam out of said first substrate with an expanded exit pupil in one ortwo dimensions, and propagating said output optical beam through asecond substrate, said second substrate being positioned substantiallyparallel to the first substrate and in juxtaposed spaced relationshiphereto; receiving a portion of said input optical beam which propagatesthrough said first substrate into the second substrate, by a furtherin-coupling diffractive element disposed on a further first or a furthersecond surface of the second substrate substantially in alignment withsaid in-coupling diffractive element and having further periodic linesparallel to said periodic lines of the in-coupling diffractive element,wherein the further second surface is opposite to the further firstsurface; diffracting optical rays of the portion of the input opticalbeam, which are incident from said another half space relative to saidplane, using said further in-coupling diffractive element to provide afurther one diffracted optical beam substantially within said furtherfirst and further second surfaces such that at least a portion of saidfurther one diffracted optical beam is coupled only to one of said oneor more further out-coupling diffractive elements which is opposite tosaid one of the one or more out-coupling diffractive elements; andcoupling by diffraction parts of said further one diffracted opticalbeam from the second substrate using said one of said one or morefurther out-coupling diffractive elements for providing a further outputoptical beam out of said second substrate with said expanded exit pupilin said one or two dimensions.
 19. The method of claim 18, furthercomprising: diffracting optical rays of the input optical beam, whichare incident from another half space relative to said plane, to provideanother diffracted optical beam substantially within said first andsecond surfaces such that at least a portion of said another diffractedoptical beam is coupled only to another of said one or two out-couplingdiffractive elements, wherein said another of said one or moreout-coupling diffractive elements disposed on the first or the secondsurface of the first substrate; coupling by diffraction parts of saidanother diffracted optical beam from the first substrate using anotherof said one or more out-coupling diffractive elements for providinganother output optical beam out of said first substrate with an expandedexit pupil in one or two dimensions, and propagating said another outputoptical beam through the second substrate; diffracting optical rays ofsaid portion of the input optical beam, which are incident from said onehalf space relative to said plane, using said further in-couplingdiffractive element to provide a further another diffracted optical beamsubstantially within said further first and further second surfaces suchthat at least a portion of said further another diffracted optical beamis coupled only to another of the one or more further out-couplingdiffractive elements which is opposite to said another of the one ormore out-coupling diffractive elements; and coupling by diffractionparts of said further another diffracted optical beam from the secondsubstrate using said another of said one or more further out-couplingdiffractive elements for providing a further another output optical beamout of said second substrate with said expanded exit pupil in said oneor two dimensions.
 20. The method of claim 19, wherein the one andanother of said one or more out-coupling diffractive elements aresymmetrical relative to the in-coupling diffractive element and the oneand another of the one or more further out-coupling diffractive elementsare symmetrical relative to the further in-coupling diffractive element.21. The method of claim 19, wherein distances from the one and anotherof said one or more out-coupling diffractive elements to the in-couplingdiffraction element are different than corresponding distances from theone and another of the one or more further out-coupling diffractiveelements to the further in-coupling diffractive element.
 22. The methodof claim 1, wherein, when using identical materials and identicalsurrounding material for the first and second substrates, thein-coupling diffractive element has a period of the periodic lines equalto a maximum wavelength of the input optical beam divided by an index ofrefraction of the first and second substrates, and the furtherin-coupling diffractive element has a further period of the furtherperiodic lines equal to a minimum wavelength of the input optical beamdivided by an index of refraction of a surrounding material of the firstand second substrates, or when using identical materials and identicalsurrounding material for the first and second substrates, the furtherin-coupling diffractive element has a further period of the furtherperiodic lines equal to a maximum wavelength of the input optical beamdivided by an index of refraction of the first and second substrates,and the in-coupling diffractive element has a period of the periodiclines equal to a minimum wavelength of the input optical beam divided byan index of refraction of a surrounding material of the first and secondsubstrates.
 23. The method of claim 1, wherein the two or moreout-coupling diffractive elements, the in-coupling diffractive element,the one or more further out-coupling diffractive elements and thefurther in-coupling diffractive element have parallel periodic lines.24. An electronic device, comprising: a data processing unit; an opticalengine operatively connected to the data processing unit for receivingimage data from the data processing unit; a display device operativelyconnected to the optical engine for forming an image based on the imagedata; and an exit pupil expander comprising: a first substrate ofoptical material having a first surface and a second surface which isopposite to the first surface; an in-coupling diffractive elementdisposed on the first or the second surface of said first substrate; oneor more out-coupling diffractive elements disposed on the first or thesecond surface, wherein said in-coupling diffractive element isconfigured to diffract optical rays of the input optical beam, which areincident from one half space relative to a plane perpendicular to saidfirst substrate and parallel to periodic lines of the in-couplingdiffractive element, so as to provide one diffracted optical beamsubstantially within said first and second surfaces such that at least aportion of said one diffracted optical beam is coupled only to one ofsaid one or more out-coupling diffractive elements; a second substrateof optical material having a further first surface and a further secondsurface and being positioned substantially parallel to the firstsubstrate and in juxtaposed spaced relationship hereto, wherein thefurther second surface is opposite to the further first surface; afurther in-coupling diffractive element disposed on the further first orthe further second surface substantially in alignment with saidin-coupling diffractive element and having further periodic linesparallel to said periodic lines of the in-coupling diffractive element,and configured to receive a portion of said input optical beam whichpropagates through said first substrate into the further substrate; andone or more further out-coupling diffractive elements disposed on thefurther first or the further second surface and each being substantiallyopposite to a corresponding diffractive element of said one or moreout-coupling diffractive elements, wherein said further in-couplingdiffractive element is configured to diffract optical rays of theportion of the input optical beam, which are incident from another halfspace relative to said plane, to provide a further one diffractedoptical beam substantially within said further first and further secondsurfaces such that at least a portion of said further diffracted opticalbeam is coupled only to one of said one or more further out-couplingdiffractive elements which is opposite to said one of the one or moreout-coupling diffractive elements, and wherein said one of the one ormore out-coupling diffractive elements is configured to couple bydiffraction parts of said one diffracted optical beam from the firstsubstrate for providing an output optical beam out of said firstsubstrate with an expanded exit pupil in one or two dimensions, saidoutput optical beam being propagated through said second substrate, andsaid one of the one or more further out-coupling diffractive elements isconfigured to couple by diffraction parts of said further one diffractedoptical beams from the second substrate for providing a further outputoptical beam out of said second substrate with said expanded exit pupilin said one or two dimensions.
 25. The electronic device of claim 24,wherein said in-coupling diffractive element is further configured todiffract optical rays of the input optical beam, which are incident fromanother half space relative to said plane, to provide another diffractedoptical beam substantially within said first and second surfaces suchthat at least a portion of said another diffracted optical beam iscoupled only to another of said one or two out-coupling diffractiveelements, wherein said further in-coupling diffractive element isconfigured to diffract optical rays of said portion of the input opticalbeam, which are incident from said one half space relative to saidplane, to provide a further another diffracted optical beamsubstantially within said further first and further second surfaces suchthat at least a portion of said further another diffracted optical beamis coupled only to another of said one or two further out-couplingdiffractive elements which is opposite to said another of the one ormore out-coupling diffi active elements, wherein said one and another ofthe one or more out-coupling diffractive elements are configured tocouple by diffraction parts of said one and another diffracted opticalbeams from the first substrate for providing two output optical beamsout of said first substrate with the expanded exit pupil in one or twodimensions, said output optical beams being propagated through saidsecond substrate, and said one and another of the further one or morediffractive elements are configured to couple by diffraction parts ofsaid further one and further another diffracted optical beams from thesecond substrate for providing two further output optical beams out ofsaid second substrate with the expanded exit pupil in said one or twodimensions.